Intraneuronal β-amyloid (Aβ) factors significantly in the early pathogenesis of Alzheimer's disease (AD), (Gouras, G. K., Almeida, C. G., and Takahashi, R. H., 2005, Intraneuronal Abeta accumulation and origin of plaques in Alzheimer's disease, Neurobiol Aging 26:1235-1244; Gouras, G. K., Tsai, J., Naslund, J., Vincent, B., Edgar, M., Checler, F., Greenfield, J. P., Haroutunian, V., Buxbaum, J. D., Xu, H., et al., 2000, Intraneuronal Abeta42 accumulation in human brain, Am J Pathol 156:15-20; Hartman, T., 2005, Cholesterol and Alzheimer's disease: statins, cholesterol depletion in APP processing and Abeta generation, Subcell Biochem 38:365-380; and LaFerla, F. M., Green, K. N., and Oddo, S., 2007, Intracellular amyloid-beta in Alzheimer's disease, Nat Rev Neurosci 8:499-509) which historically is more recognized for the occurrence of extracellular plaques comprised of Aβ42, ubiquitin and numerous chaperones. Inclusion body myositis (IBM), another disorder associated with intracellular Aβ deposits, is a major cause of skeletal muscle inflammation and degeneration in the elderly. Cytosolic Aβ has been shown to induce programmed cell death (apoptosis) in a number of experimental and transgenic models involving several cell types (LaFerla, F. M., Green, K. N., and Oddo, S., 2007, Intracellular amyloid-beta in Alzheimer's disease, Nat Rev Neurosci 8:499-509; Magrane, J., Rosen, K. M., Smith, R. C., Walsh, K., Gouras, G. K., and Querfurth, H. W., 2005, Intraneuronal beta-amyloid expression downregulates the Akt survival pathway and blunts the stress response, J Neurosci 25:10960-10969; Oakley, H., Cole, S. L., Logan, S., Maus, E., Shao, P., Craft, J., Guillozet-Bongaarts, A., Ohno, M., Disterhoft, J., Van Eldik, L., et al., 2006, Intraneuronal beta-amyloid aggregates, neurodegeneration, and neuron loss in transgenic mice with five familial Alzheimer's disease mutations: potential factors in amyloid plaque formation, J Neurosci 26:10129-10140; Querfurth, H. W., Suhara, T., Rosen, K. M., McPhie, D. L., Fujio, Y., Tejada, G., Neve, R. L., Adelman, L. S., and Walsh, K., 2001, Beta-amyloid peptide expression is sufficient for myotube death: implications for human inclusion body myopathy, Mol Cell Neurosci 17:793-810; Link, C. D., 1995, Expression of human beta-amyloid peptide in transgenic Caenorhabditis legans, Proc Natl Acad Sci USA 92:9368-9372; and Zhang, Y., McLaughlin, R., Goodyer, C., and LeBlanc, A., 2002, Selective cytotoxicity of intracellular amyloid beta peptide1-42 through p53 and Bax in cultured primary human neurons, J Cell Biol 156:519-529). The serine-threonine kinase Akt maintains post-mitotic cell viability through phosphorylation of pro-apoptotic mediators, thereby inactivating them. These factors include the transcription factor forkhead (FOXO), the tau kinase GSK-3β, and the Bcl2 antagonist BAD proteins (Brunet, A., Bonni, A., Zigmond, M. J., Lin, M. Z., Juo, P., Hu, L. S., Anderson, M. J., Arden, K. C., Blenis, J., and Greenberg, M. E., 1999, Akt promotes cell survival by phosphorylating and inhibiting a Forkhead transcription factor, Cell 96:857-868; Cross, D. A., Alessi, D. R., Cohen, P., Andjelkovich, M., and Hemmings, B. A., 1995, Inhibition of glycogen synthase kinase-3 by insulin mediated by protein kinase B, Nature 378:785-789; Datta, S. R., Dudek, H., Tao, X., Masters, S., Fu, H., Gotoh, Y., and Greenberg, M. E., 1997, Akt phosphorylation of BAD couples survival signals to the cell-intrinsic death machinery, Cell 91:231-241; and Zheng, W. H., Kar, S., and Quirion, R., 2000, Insulin-like growth factor-1-induced phosphorylation of the Forkhead family transcription factor FKHRL1 is mediated by Akt kinase in PC12 cells, J Biol Chem 275:39152-39158). Conversely, dephosphorylation of Akt decreases its activity, derepresses pro-apoptotic proteins and results in the sensitization of the cell to environmental stressors and initiation of processes leading to death (Gao, T., Furnari, F., and Newton, A. C., 2005, PHLPP: a phosphatase that directly dephosphorylates Akt, promotes apoptosis, and suppresses tumor growth, Mol Cell 18:13-24; and Ugi, S., Imamura, T., Maegawa, H., Egawa, K., Yoshizaki, T., Shi, K., Obata, T., Ebina, Y., Kashiwagi, A., and Olefsky, J. M., 2004, Protein phosphatase 2A negatively regulates insulin's metabolic signaling pathway by inhibiting Akt (protein kinase B) activity in 3T3-L1 adipocytes, Mol Cell Biol 24:8778-8789). Akt has multiple additional metabolic and trophic actions, such as the stimulation of the glucose transporter (glut-4), on mitochondrial function and synaptic plasticity (Horwood, J. M., Dufour, F., Laroche, S., and Davis, S., 2006, signaling mechanisms mediated by the phosphoinositide 3-kinase/Akt cascade in synaptic plasticity and memory in the rat, Eur J Neurosci 23:3375-3384; Tapodi, A., Debreceni, B., Hanto, K., Bognar, Z., Wittmann, I., Gallyas, F., Jr., Varbiro, G., and Sumegi, B., 2005, Pivotal role of Akt activation in mitochondrial protection and cell survival by poly(ADP-ribose)polymerase-1 inhibition in oxidative stress, J Biol Chem 280:35767-35775; and Uchiyama, T., Engelman, R. M., Maulik, N., and Das, D. K., 2004, Role of Akt signaling in mitochondrial survival pathway triggered by hypoxic preconditioning, Circulation 109:3042-3049).
Interference with or alteration of the Akt signaling pathway has emerged as an important feature in several neurodegenerative diseases characterized by neuronal attrition including AD and schizophrenia (Griffin, R. J., Moloney, A., Kelliher, M., Johnston, J. A., Ravid, R., Dockery, P., O'Connor, R., and O'Neill, C., 2005, Activation of Akt/PKB, increased phosphorylation of Akt substrates and loss and altered distribution of Akt and PTEN are features of Alzheimer's disease pathology, J Neurochem 93:105-117; Pei, J. J., Khatoon, S., An, W. L., Nordlinder, M., Tanaka, T., Braak, H., Tsujio, I., Takeda, M., Alafuzoff, I., Winblad, B., et al., 2003, Role of protein kinase B in Alzheimer's neurofibrillary pathology, Acta Neuropathol (Berl) 105:381-392; and Rickle, A., Bogdanovic, N., Volkman, I., Winblad, B., Ravid, R., and Cowburn, R. F., 2004, Akt activity in Alzheimer's disease and other neurodegenerative disorders, Neuroreport 15:955-959.
The PI3K-Akt signaling pathway is a major site of control for numerous cellular response mechanisms to environmental stress, and growth and differentiation signals. This pathway is pivotally affected in the opposing processes of tumorigenesis and apoptosis (Martelli, A. M., Faenza, I., Billi, A. M., Manzoli, L., Evangelisti, C., Fala, F., and Cocco, L., 2006, Intranuclear 3′-phosphoinositide metabolism and Akt signaling: new mechanisms for tumorigenesis and protection against apoptosis? Cell Signal 18:1101-1107; and Asano, T., Yao, Y., Shin, S., McCubrey, J., Abbruzzese, J. L., and Reddy, S. A., 2005, Insulin receptor substrate is a mediator of phosphoinositide 3-kinase activation in quiescent pancreatic cancer cells, Cancer Res 65:9164-9168). Once stimulated, insulin- and IGF-receptor tyrosine kinases next phosphorylate insulin receptor substrate (IRS), which then initiates the PI3K-Akt signaling cascade (Myers, M. G., Jr., Sun, X. J., and White, M. F., 1994, The IRS-1 signaling system, Trends Biochem Sci 19:289-293; Shpakov, A. O., and Pertseva, M. N., 2000, Structural and functional characterization of insulin receptor substrate proteins and the molecular mechanisms of their interaction with insulin superfamily tyrosine kinase receptors and effector proteins, Membr Cell Biol 13:455-484; and Andjelkovic, M., Alessi, D. R., Meier, R., Fernandez, A., Lamb, N. J., Frech, M., Cron, P., Cohen, P., Lucocq, J. M., and Hemmings, B. A., 1997, Role of translocation in the activation and function of protein kinase B, J Biol Chem 272:31515-31524). PI3K activation, in turn, results in the 3′-phosphorylation of second messenger, membrane-bound signaling inositol lipids. These lipids (e.g., PtdIns P3 (PIP3)) bring together PDK and Akt in a sub-membrane complex through interaction with each of their pleckstrin homology (PH) domains (Brunet, A., Datta, S. R., and Greenberg, M. E., 2001, Transcription-dependent and independent control of neuronal survival by the PI3K-Akt signaling pathway, Curr Opin Neurobiol 11:297-305). After sequential activation by PDK and another kinase, Akt phosphorylates a number of cellular targets, regulating their function (Alessi, D. R., James, S. R., Downes, C. P., Holmes, A. B., Gaffney, P. R., Reese, C. B., and Cohen, P., 1997, Characterization of a 3-phosphoinositide-dependent protein kinase which phosphorylates and activates protein kinase B alpha, Curr Biol 7:261-269).
Akt is activated through phosphorylations at Thr308 in the catalytic domain and Ser473 in the regulatory domain upon translocation from cytosol to the plasma membrane. PhosphoThr308 is essential while Ser 473 phosphorylation is required for full activation of Akt. Phosphoinositide-dependent protein kinase 1 (PDK1) was identified as the protein kinase responsible for the phosphorylation of Thr308 on Akt (Stokoe, D., Stephens, L. R., Copeland, T., Gaffney, P. R., Reese, C. B., Painter, G. F., Holmes, A. B., McCormick, F., and Hawkins, P. T., 1997, Dual role of phosphatidylinositol-3,4,5-trisphosphate in the activation of protein kinase B, Science 277:567-570; and Balendran, A., Casamayor, A., Deak, M., Paterson, A., Gaffney, P., Currie, R., Downes, C. P., and Alessi, D. R., 1999, PDK1 acquires PDK2 activity in the presence of a synthetic peptide derived from the carboxyl terminus of PRK2, Curr Biol 9:393-404). The protein kinase candidates for the Ser473 phosphorylation include: MAPKAP kinase-2 (PDK2), protein kinase Ca (PKCa) isoforms, integrin-linked kinase (ILK), DNA-dependent protein kinase (DNA-PK), ataxia telangiectasia mutated (ATM) gene product, mammalian target of rapamycin (mTOR), PDK1 itself, a still unknown kinase or an autophosphorylation event (Bayascas, J. R., and Alessi, D. R., 2005, Regulation of Akt/PKB Ser473 phosphorylation, Mol Cell 18:143-145; Feng, J., Park, J., Cron, P., Hess, D., and Hemmings, B. A., 2004, Identification of a PKB/Akt hydrophobic motif Ser473 kinase as DNA-dependent protein kinase, J Biol Chem 279:41189-41196; Leslie, N. R., Biondi, R. M., and Alessi, D. R., 2001, Phosphoinositide-regulated kinases and phosphoinositide phosphatases, Chem Rev 101:2365-2380; Sarbassov, D. D., Guertin, D. A., Ali, S. M., and Sabatini, D. M., 2005, Phosphorylation and regulation of Akt/PKB by the rictor-mTOR complex, Science 307:1098-1101; Toker, A., and Newton, A. C., 2000, Akt/protein kinase B is regulated by autophosphorylation at the hypothetical PDK-2 site, J Biol Chem 275:8271-8274; and Hresko, R. C., and Mueckler, M., 2005, mTOR.RICTOR is the Ser473 kinase for Akt/protein kinase B in 3T3-L1 adipocytes, J Biol Chem 280:40406-40416). The lack of consensus is illustrated by one study in 3T3-L1 adipocytes in which depletion of DNA-PK, ATM, or ILK had no effect on insulin-stimulated Akt Ser473 phosphorylation, whereas the depletion of Rictor resulted in inhibition (de la Monte, S. M., Tong, M., Lester-Coll, N., Plater, M., Jr., and Wands, J. R., 2006, Therapeutic rescue of neurodegeneration in experimental type 3 diabetes: relevance to Alzheimer's disease, J Alzheimers Dis 10:89-109).
Previous studies of the effects of intracellular β-amyloid on this pathway have suggested that intraneuronal Aβ1-42 expression leads to a sequential decrease in the levels of p-Akt, an increase in activation of GSK-3β, and induction of apoptosis (Magrane, J., Rosen, K. M., Smith, R. C., Walsh, K., Gouras, G. K., and Querfurth, H. W., 2005, Intraneuronal beta-amyloid expression downregulates the Akt survival pathway and blunts the stress response, J Neurosci 25:10960-10969).
There is widening recognition that AD is closely linked to a relative state of insulin resistance in the brain, so-called type III diabetes (Messier, C., and Teutenberg, K., 2005, The role of insulin, insulin growth factor, and insulin-degrading enzyme in brain aging and Alzheimer's disease, Neural Plast 12:311-328). Levels of insulin-like growth factor I (IGF-I), insulin and cognate receptors are significantly dysregulated in AD brain (Steen, E., Terry, B. M., Rivera, E. J., Cannon, J. L., Neely, T. R., Tavares, R., Xu, X. J., Wands, J. R., and de la Monte, S. M., 2005, Impaired insulin and insulin-like growth factor expression and signaling mechanisms in Alzheimer's disease—is this type 3 diabetes? J Alzheimers Dis 7:63-80; and Hoyer, S., 2004, Causes and consequences of disturbances of cerebral glucose metabolism in sporadic Alzheimer disease: therapeutic implications, Adv Exp Med Biol 541:135-152). In normal brain, IGF-I and insulin promote glucose utilization, energy metabolism and neuronal survival (Bondy, C. A., and Cheng, C. M., 2004, Signaling by insulin-like growth factor 1 in brain, Eur J Pharmacol 490:25-31), in large part through PI3K/Akt/GSK-30 signaling (Abbott, M. A., Wells, D. G., and Fallon, J. R., 1999, The insulin receptor tyrosine kinase substrate p58/53 and the insulin receptor are components of CNS synapses, J Neurosci 19:7300-7308). Insulin receptors populate neuronal synapses and astrocytes in memory-processing brain regions (Lee, C. C., Huang, C. C., Wu, M. Y., and Hsu, K. S., 2005, Insulin stimulates postsynaptic density-95 protein translation via the phosphoinositide 3-kinase-Akt-mammalian target of rapamycin signaling pathway, J Biol Chem 280:18543-18550). Acute insulin treatment increased memory function in rats on a passive-avoidance task (Park, C. R., Seeley, R. J., Craft, S., and Woods, S. C., 2000, Intracerebroventricular insulin enhances memory in a passive-avoidance task, Physiol Behav 68:509-514) and in small studies involving normal adults and AD patients (Kern, W., Peters, A., Fruehwald-Schultes, B., Deininger, E., Born, J., and Fehm, H. L., 2001, Improving influence of insulin on cognitive functions in humans, Neuroendocrinology 74:270-280; Zhao, L., Teter, B., Morihara, T., Lim, G. P., Ambegaokar, S. S., Ubeda, O. J., Frautschy, S. A., and Cole, G. M., 2004, Insulin-degrading enzyme as a downstream target of insulin receptor signaling cascade: implications for Alzheimer's disease intervention, J Neurosci 24:11120-11126; and Ho, L., Qin, W., Pompl, P. N., Xiang, Z., Wang, J., Zhao, Z., Peng, Y., Cambareri, G., Rocher, A., Mobbs, C. V., et al., 2004. Diet-induced insulin resistance promotes amyloidosis in a transgenic mouse model of Alzheimer's disease, Faseb J 18:902-904).
Primary hippocampal neurons treated with insulin show an inductive effect on insulin degrading enzyme (IDE) protein levels. The feed forward effect is mediated by PI3K/Akt (Zhao, L., Teter, B., Morihara, T., Lim, G. P., Ambegaokar, S. S., Ubeda, O. J., Frautschy, S. A., and Cole, G. M., 2004, Insulin-degrading enzyme as a downstream target of insulin receptor signaling cascade: implications for Alzheimer's disease intervention, J Neurosci 24:11120-11126). IDE is a metalloprotease enzyme also held responsible for Aβ monomer degradation. Thus, IDE deficiency (IDE −/− mice) resulted in a decrease□ degradation in both brain membrane fractions and primary neuronal cultures and in the cerebral accumulation of Aβ (Farris, W., Mansourian, S., Chang, Y., Lindsley, L., Eckman, E. A., Frosch, M. P., Eckman, C. B., Tanzi, R. E., Selkoe, D. J., and Guenette, S., 2003, Insulin-degrading enzyme regulates the levels of insulin, amyloid beta-protein, and the beta-amyloid precursor protein intracellular domain in vivo, Proc Natl Acad Sci USA 100:4162-4167). Similarly, insulin resistance and IDE deficiencies created in Tg2576 mice fed and oil-enriched diet, or one high in fat, were associated with increased Aβ monomer buildup and plaque burden (Zhao, L., Teter, B., Morihara, T., Lim, G. P., Ambegaokar, S. S., Ubeda, O. J., Frautschy, S. A., and Cole, G. M., 2004, Insulin-degrading enzyme as a downstream target of insulin receptor signaling cascade: implications for Alzheimer's disease intervention, J Neurosci 24:11120-11126; and Ho, L., Qin, W., Pompl, P. N., Xiang, Z., Wang, J., Zhao, Z., Peng, Y., Cambareri, G., Rocher, A., Mobbs, C. V., et al., 2004, Diet-induced insulin resistance promotes amyloidosis in a transgenic mouse model of Alzheimer's disease, Faseb J 18:902-904).
From knock-down models that test the IGF-I, IR and IRS axis, the loss of insulin signaling is expected to increase tau phosphorylation at AD-relevant GSK-3β and cdk5 sites (Cheng, C. M., Reinhardt, R. R., Lee, W. H., Joncas, G., Patel, S. C., and Bondy, C. A., 2000, Insulin-like growth factor 1 regulates developing brain glucose metabolism, Proc Natl Acad Sci USA 97:10236-10241; and Schubert, M., Gautam, D., Surjo, D., Ueki, K., Baudler, S., Schubert, D., Kondo, T., Alber, J., Galldiks, N., Kustermann, E., et al., 2004, Role for neuronal insulin resistance in neurodegenerative diseases, Proc Natl Acad Sci USA 101:3100-3105) and impair insulin-mediated inhibition of apoptosis as well as the stimulation of glucose uptake. Animal models in which brain insulin is depleted by intracerebral streptozotocin also have loss of the same insulin signaling components and show neurodegenerative changes in common with AD (Lester-Coll, N., Rivera, E. J., Soscia, S. J., Doiron, K., Wands, J. R., and de la Monte, S. M., 2006, Intracerebral streptozotocin model of type 3 diabetes: relevance to sporadic Alzheimer's disease, J Alzheimers Dis 9:13-33). Although the results argue for activation of insulin signaling in AD therapeutics, chronic insulin stimulation may have negative consequences such as the development of peripheral insulin resistance and the accumulation of Aβ through competition for a limited pool of IDE in AD brain.
Therefore, different targets should be sought.